Hi Russell. Have you had a look at Scratch? It's been developed by MIT specifically for kids and is very visual and has an appealing entry-level approach.
It's free and has a busy online community who trade bits of programming etc back and forth. My 8 year old girl has done a bit with it (used it particularly to do small animations and dramas) but DH has done some more sophisticated gaming stuff with it, although it started to bug when he got too complex with it.
Certainly worth a look though - it may suit your son.